This invention relates to starting of high intensity discharge lamps and, more particularly, to new and improved methods and apparatus for providing high voltage, short duration pulses for starting of high intensity discharge lamps.
High intensity discharge lamps, such as high pressure sodium lamps, commonly include noble gases at pressures below 100 torr. Lamps containing nobel gases at pressures below 100 torr can be started and operated by utilizing an igniter in conjunction with a lamp ballast. The lamp ballast converts the ac line voltage to the proper amplitude and impedance level for lamp operation. The igniter provides pulses which assist in initiating discharge. The igniter is a relatively large and heavy circuit and is typically built into or located near the lamp ballast.
It has been found that the inclusion in high pressure sodium lamps of xenon as the nobel gas at pressures well in excess of 100 torr is beneficial to lamp performance. However, the pulse energy requirements for starting of the discharge lamp increase as the pressure of the xenon included within the lamp increases and the conventional igniter described above does not, by itself, produce reliable starting.
A conductor wrapped around the discharge tube and connected to one of the electrodes, to assist starting of a lamp containing xenon at pressures of up to 300 torr, is disclosed in U.S. Pat. No. 4,179,640 issued Dec. 18, 1979 to Larson et al. According to Larson et al., the lamp is operated from a conventional ballast and a starting pulse generator or igniter is utilized. Another arrangement for starting high pressure discharge lamps is shown in U.S. Pat. No. 4,137,483 issued Jan. 30, 1979 to Ochi et al. A switching circuit contained within the lamp induces a high voltage starting pulse which operates in conjunction with a conductor wrapped around the discharge tube to initiate discharge in the lamp. The pressure of the noble gas in the lamp is not specified and is presumed to be relatively low, since starting voltages ranging from 2 to 3 kilovolts are used.
It is desirable to provide a simple, low cost starting circuit which can be included within the outer jacket of the lamp and which does not utilize a pulse transformer. Elimination of a pulse transformer results in a considerable saving in mass and volume of the starting circuit. Furthermore, the pulse rise time and pulse width are not limited to relatively large values by the inductive reactance of such transformers.
A device capable of storing electrical energy and, upon momentary short circuiting of a pair of terminals, of providing a high amplitude pulse is a spiral line pulse generator disclosed by R. A. Fitch et al. in U.S. Pat. No. 3,389,015 issued Nov. 29, 1966. While the device disclosed therein is a simple wound capacitor, it can, when properly utilized, provide the dual functions of storage and voltage multiplication. The spiral line pulse generator is a transient field reversal device which provides a roughly triangular pulse. Its peak voltage is a multiple of the initial charging voltage.